The overall objective of our program is to study the regulation of gene expression during the cell cycle. Predicated on previous results from this laboratory which indicate that in HeLa S3 cells histone genes are transcribed and translated exclusively during the S phase of the cell cycle we have been focusing our attention on the regulation of this set of genetic sequences. During the initial phase of this program we synthesized a 3H-labeled single stranded DNA complementary to the 5 histone mRNA's. This high resolution probe was utilized in studies which demonstrated: a) histone messenger RNA sequences are present in the polyribosomes during S phase but not during the G1 phase of the cell cycle; b) histone messenger RNA transcription was restricted to chromatin from S phase cells; and c) S phase nonhistone chromosomal proteins dictate the availability of histone genes for transcription. During the next year we plan to fractionate histone messenger RNA's into the five components and utilize these individual mRNA's for synthesis of complementary DNA's. The complementary DNA's will be utilized in studies directed towards defining the regulation of individual histone genes and the in vivo processing of histone gene transcripts. BIBLIOGRAPHIC REFERENCES: Stein, J.l., Thrall, C.L., Park, W.D., Mans, R.J. and Stein, G.S. "Hybridization Analysis of Histone Messenger RNA: Association with Polyribosomes During the Cell Cycle", Science 189:557-558 (1975). Park, W.D., Thrall, C.L., Stein, J.L. & Stein, G.S. "Activation of Histone Gene Transcription from Chromatin of G1 HeLa Cells by S Phase Nonhistone Chromosomal Proteins", FEBS Lett. 62:226-229 (1976).